A satisfactory ink for printing upon polymeric films, such as regenerated cellulose films used in sausage casings, should cure to form a tack-free printed impression with good adhesion and abrasion resistance, and, yet, the cured impression should be flexible such that it will not crack upon flexing, wrinkling or stretching of the film. A satisfactory ink should also have a rapid curing rate, since methods of processing and handling sausage casings are quite rapid. Therefore, a rapid curing rate of the ink is desirable to be compatible with these high-speed handling and processing methods.
An example of such an ink for use upon such polymeric films is that disclosed in U.S. Pat. No. 3,316,189 to G. M. Adams. Disclosed is an ink composition, curable by the application of heat, that comprises a polyol prepolymer, such as polyester, polyether or castor polyols, an isocyanate prepolymer prepared from a polyisocyanate, a polymeric resin hardener, a pigment, and a volatile organic solvent.
With this ink, as with a number of other heat curable inks, the presence of the volatile organic solvent is undesirable, because, upon curing of the ink, this solvent volatilizes into the atmosphere creating possible health and safety problems. Efforts have been made to solve this problem by reducing or even eliminating the volatile solvents. A problem associated with this approach, however, is an increase in viscosity of the composition causing difficulties in handling and application. It is, therefore, desirable to produce a composition of proper viscosity that is free of volatile solvents. It is also desirable that the composition be curable using a minimal amount of energy.
Ink compositions have been developed that are free of volatile solvents, are curable with a low amount of energy by ultraviolet light, and have a low viscosity. However these compositions, being directed mainly to printing on paper or metal substrates have generally not been entirely suitable for use with flexible polymeric films such as those used for sausage casings. Although many of these compositions cure rapidly and are suitable for high-speed processing requirements, the printed impression formed by the cured ink is often brittle and is subject to cracking when the casing is folded or stretched. Other ultraviolet light curable ink compositions show greater flexibility and will cure to form impressions that will not crack. However. these compositions generally have unsuitably slow curing rates and/or form tacky impressions with poor adhesion to the polymeric film substrate. Polymeric films, such as the cellulosic type used in food packaging require that the impression be flexible, and have good adhesion. The impression must be fully cured and not be tacky or rub off. It is generally the case that an ink composition that cures more rapidly and forms nontacky impressions of greater adherence is often more brittle and less flexible.
Thus, an ink composition having the conflicting and often exclusive properties of rapid cure under ultraviolet light and formation of an impression having physical properties suitable for use with flexible films, such as sausage casings, would be highly desirable. To form such an ink is very difficult, because of the many unpredictable factors associated with ink manufacture. The components of the ink will often interact in an unpredictable manner to adversely affect the properties of the ink such as, for example, the cure rate, and/or the physical properties relating to handling of the ink, such as viscosity and shelf life.
It is, therefore, an object of the invention to provide an ultraviolet curable ink composition that possesses the combination of properties to make it suitable for use for printing on flexible films.
SUMMARY OF THE lNVENTION
It has now been found that an ink composition can be made that is free of volatile solvents, is rapidly curable with ultraviolet light, and forms an impression oF suitable physical properties for use on flexible polymeric films such as sausage casings.
The invention in one aspect is an uItraviolet light curable ink composition which comprises
(i) a first urethane comprising a polyol urethane which is the reaction product of about one equivalent of an isocyanate containing intermediate, the isocyanate intermediate being the reaction product of about one equivalent of a monomeric polyol and about two equivalents of an organic polyisocyanate, and one about equivalent of a monomeric olefinically unsaturated compound containing exactly one active hydrogen, and PA1 (ii) a second urethane comprising an acrylate-capped polycaprolactone defined by the formula; ##STR1## where Z is hydrogen or methyl; Q is the residue remaining after reaction of the caprolactone polyol with the isocyanato and acrylyl compound, hereinafter more fully described; R is a linear or branched divalent alkylene having from about 2 to about 5 carbon atoms; G is the polyvalent residue remaining after reaction of a substituted or unsubstituted polyisocyanate with the caprolactone polyol and acrylyl compound and can be a linear or branched alkylene having from 1 to about 10 carbon atoms, or arylene, alkarylene and aralkylene having from about 6 to about 12 carbon atoms, cycoalkylene having from about 5 to about 10 carbon atoms, or bicycloalkylene having from about 7 to about 15 carbon atoms; and x is an integer having a value from 1 to 4; PA1 (iii) an ultraviolet photoinitiator in an amount sufficient to cure the ink composition when exposed to ultraviolet light, and PA1 (iv) a pigment; with the proviso that the first urethane and the second urethane are present in an amount such that the weight ratio of the first urethane to the second urethane is from about 2:5 to about 3:2, preferably about 3:5 to about 5:5.
Another aspect of the invention is an ultraviolet curable ink comprising a first urethane which comprises the reaction product of an acrylated epoxidized drying oil and an organic isocyanate; a second urethane as defined above, an ultraviolet photo-initiator, and a pigment such that the first urethane and the second urethane are present in the weight ratios described above.
It is preferable to add a reactive diluent monomer, more fully described below, to provide a suitable viscosity for use as an ink.
The combined amounts of the first urethane and the second urethane should preferably be about from about 20 weight percent to about 80 weight percent, most preferably about 50 weight percent to about 70 weight percent. The reactive diluent monomer, if used, should be present in an amount from about 2 weight percent to about 30 weight percent, preferably about 5 to about 20 weight percent. The ultraviolet photoinitiator should be present in an amount from about 1 weight percent to about 10 weight percent, preferably about 3 weight percent to about 7 weight percent. The pigment should be present in an amount greater than 0 weight percent up to about 70 weight percent, preferably about 10 weight percent to about 50 weight percent. The above weight percentages are based upon the total weight of the ink composition.
The active hydrogen atoms referred to herein are those which display activity according to the Zerewitinoff test as described by Kohler, J. Am. Chem. Soc. 49, 3181 (1927).